1. Field of the Invention
This invention relates to wood preservative compositions and to a method for their manufacture.
2. Brief Description of the Prior Art
Compositions containing, as active ingredients, compounds of copper, of chromium and of arsenic, hereafter called copper/chrome/arsenic compositions, are in widespread use as wood preservatives. Such compositions are required to be water-soluble and are generally made available as powder, as slurries of solid and solution, or as concentrated solutions, each for dissolution and/or dilution in water before use.
Salt type copper/chrome/arsenic wood preservative compositions are in common use. Such compositions may include ingredients in salt form the reaction of which may result in a content of "inert" ingredients such as, for example, sodum sulphate.
Examples of certain salt-type copper/chrome/arsenic wood preservative compositions are described in British Standard No. 4072 That Standard specifies two formulations identified as Type 1 and Type 2 containing mixtures of copper sulphate, sodium dichromate or potassium dichromate and hydrated arsenic pentoxide. These mixtures may be made available as a mixture of dry ingredients, or as a paste mixture in water of the ingredients, or as separate parcels of the dry ingredients in the correct relative quantities in a single container. Where the ingredients are supplied separately it is essential to dissolve them in the order (a) dichromate, (b) copper sulphate and (c) arsenic pentoxide and to agitate the solution for at least 5 minutes at ambient temperature between each addition to obtain a suitable working solution. The compositions of Type 1 and Type 2 formulations as specified in the aforementioned Standard No. 4072 are as follows:
______________________________________ Type 1 Type 2 ______________________________________ Copper sulphate - expressed 32.6% 35.0% as CuSO.sub.4 5H.sub.2 O Dichromate - expressed as 41.0% 45.0% K.sub.2 CR.sub.2 O.sub.7 or Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O Arsenic- expressed as 26.4% 20.0% As.sub.2 O.sub.5.2H.sub.2 O ______________________________________
Examples of similar compositions which have been Standardised by the American Wood Preservers Association as Types A, B and C are as follows:
______________________________________ Type A Type B Type C ______________________________________ Copper sulphate - expressed 18.1 19.6 18.5 as CuO Sodium dichromate - 65.5 35.3 47.5 expressed as CrO.sub.3 Arsenic - expressed as 16.4 45.1 34.0 As.sub.2 O.sub.5 ______________________________________
It would be desirable to include copper arsenate in copper/chrome/arsenic wood preservative compositions since this is a readily produced chemical and the use as a single compound as a source of both copper and arsenic would apparently facilitate formulation. One method of preparing copper arsenate is described in U.S. Pat. No. 4,103,000 the product being, according to that patent "inexpensive and easy to prepare". The method so described comprises reacting copper metal with arsenic acid in the presence of an oxidising agent and an acid catalyst selected from the group consisting of nitric acid, hydrochloric acid and sulphuric acid. Oxygen may be sparged into the reaction mixture, preferably under pressure, as the oxidising agent, although nitric acid is the catalyst usually employed and in this case the use of a further oxidising agent is optional.
Other methods for the production of copper arsenate are available, for example the reaction of copper sulphate with sodium arsenate or arsenic acid followed by neutralisation with sodium carbonate or sodium hydroxide to precipitate the copper arsenate thereby to effect at least partial separation thereof from the sulphate ions present in the reaction medium.
Yet a further method for the production of copper arsenate is disclosed in U.S. Pat. Specification No. 2,388,496. According to that method calcium arsenate is used as the source of arsenic. Calcium arsenate occurs in nature as the mineral haidingerite.
However, to achieve the inclusion of copper arsenate in copper/chrome/arsenic wood preservative compositions it has been found necessary, according to U.S. Pat. No. 2,438,511, to include, also, chromium trioxide. According to that patent chromium trioxide is used in a substantial amount at least sufficient to maintain the copper arsenate in solution when the ingredients are added to water, for example, preferably, in an amount of 15 parts by weight of chromium trioxide to 11 parts by weight of copper arsenate. In U.S. Pat. No. 4,103,000 it is confirmed that the addition of chromic acid in the appropriate amount to copper arsenate slurries results in a rapid and complete dissolution of the copper arsenate into concentrated aqueous copper/chrome/arsenic compositions.
Chromium trioxide is an expensive ingredient and it would be of benefit to enable copper arsenate to be utilised in copper/chrome/arsenic wood preservative compositions in the absence of, or in the presence of a reduced quantity of, chromium trioxide. Sodium dichromate would be a more economic source of chromium for such compositions.
The present invention provides a process for the production of a copper/chrome/arsenic wood preservative composition utilising both copper arsenate, as a source of copper and of arsenic, and sodium dichromate as a source of chromium.